The present invention relates to a process for the production of granulated dicalcium phosphate dihydrate from fine-grain, powdered dicalcium phosphate dihydrate.
Dicalcium phosphate is used technically on a large scale as an additive for toothpastes and as a carrier material in pharmaceutical tablets. For both purposes, grains or granulates with a grain size of about 100 to 500 .mu.m are required, since they are handled without difficulty in mixing and filling apparatus, i.e., they are free-flowing and do not cause much dust. However, in the case of production by the reaction of calcium carbonate in aqueous suspension with aqueous phosphoric acid, a relatively fine-grained product results, the grain size of which lies predominantly in the range of from 1 to 50 .mu.m.
Published German Patent Application DE-2,512,099, discloses a process in which highly-concentrated suspensions of calcium carbonate and phosphoric acid are mixed in a tubular reactor at a temperature of about 90.degree. C. The resultant carbon dioxide divides the mixture into fine droplets which drop through the reactor to give dicalcium phosphate and solidify sufficiently so that the granulates formed can be collected and dried. Because of the production with the formation of large amounts of carbon dioxide, such granulates have a low bulk density and strength so that they are of only limited suitability for further working up.
Norwegian Patent Specification No. 100,875 discloses the reaction of calcium carbonate with phosphoric acid in a kneader apparatus. The products are agglomerated in a moist state, but break down into a flowable powder in the case of further working up and drying. Therefore, in German Patent Specification No. DE-2,512,099, mention of further compaction by means of granulation apparatus proves to be speculative.
German Patent Specification No. DE-1,817,127 discloses granulation fine-grained phosphates by the addition of aqueous sodium hydroxide solution and acidic calcium dihydrogen phosphates. The sodium phosphate resulting from the reaction appears to act as a binding agent which holds together the grains of calcium phosphate. Such products are admittedly suitable as "mineral animal feedstuffs," but not for the pharmaceutical industry, in which the proportion of sodium phosphate would have a detrimental effect.
In addition, published European Patent Application No. 54,333, describes a process for the production of granulated calcium phosphates, also including dicalcium phosphate dihydrate, by pressing fine particles with a size of from 1 to 75 .mu.m under high pressure to give thin plates which can subsequently be broken up or ground to give a granulate of appropriate size. The dried, pressed material is not very firm, contains a high proportion of dust and, because of the breaking up or grinding, has a very angular form which is not very suitable for further working up.
In U.S. Pat. No. 3,467,495, a process is disclosed for producing granulated calcium phosphates, also including dicalcium phosphate, by carrying out the reaction of phosphoric acid with calcium carbonate or calcium hydroxide in a kneader, using a 2.5 to 10 fold amount of a finely-divided calcium phosphate as crystallization nuclei and reaction bed. Granulates of the necessary size are apparently only obtained in low yield, since a large portion must be recycled to the process as fine-grained material or as oversized grains to be ground again. Consequently, only a small proportion of usable granulate results.